This is a warning that package directories will be moving in the repo, to avoid astonishment when you pull that change.
If you’re just building syncthing and not hacking around in the source, this won’t affect you.
I’m going to move the top level packages in the repo to instead be internal packages. This cleans up the top level and in the long run clarifies what is the public interface and what isn’t (currently nothing is). Existing pull requests will need to be rebased.
The change is currently in https://github.com/syncthing/syncthing/tree/internal-reorg.